It is often desirable to compensate for the detrimental effects of the atmosphere upon light transmitted therethrough. Phase misalignments and distortions of different portions of a light beam transmitted through the atmosphere occur because of spatial and temporal variations in the density and chemical composition of the atmosphere. These variations occur both in a cross-section (the wavefront) of the beam and along the beam's propagation direction.
More particularly, variations in the density and chemical composition of the atmosphere cause corresponding variations in the index of refraction of the atmosphere. These variations in the index of refraction cause undesirable distortion of a light beam's wavefront which tend to vary along the length of the light beam. These index of refraction variations lead to local speed of light variations, causing differences in the optical path length experienced by the light. Such distortion has detrimental effects upon light beams used in various applications, including optical communications, imaging, and weaponry.
In optical communications, atmospheric distortion can substantially limit bandwidth. In imaging, atmospheric distortion can undesirably reduce the resolution of telescopes and cameras. In directed energy weaponry, atmospheric distortion can limit the amount of power delivered within a given area on a target.
Wavefront correction can be used to compensate for atmospheric distortion. Contemporary systems for wavefront correction, particularly in high power applications, commonly use mechanical devices such as deformable mirrors. The mechanical systems are useful because the large surface areas within their active correction region are required to handle high laser powers. However, such mechanical devices tend to be undesirably bulky and heavy. Thus, they have a significantly detrimental impact upon launch and flight costs for space-based systems.
The use of spatial light modulators for wavefront correction is known. For example, a phase-only dual-frequency liquid crystal spatial light modulator can vary the phase relationships of portions of a wavefront on a pixel-by-pixel basis. In this manner, phase misalignments caused by differences in the index of refraction for the optical paths traveled by different portions of a light beam can be mitigated.
However, in order to use a spatial light modulator in such an application, its effect upon the light beam must be monitored. The amount of wavefront correction needed for each portion of the light beam tends to vary generally continuously. Thus, each pixel of the spatial light modulator needs to react generally continuously to provide the desired correction.
The light beam can be monitored to facilitate such real-time control of the spatial light modulator. That is, the relative phase relationships of the different portions of the light beam can be measured and control signals can be provided to the spatial light modulator to vary the phase delays introduced thereby on a pixel-by-pixel basis.
Such precise control of the spatial light modulator requires that a phase measurement system be capable of distinguishing the amount of phase correction applied by each pixel of the spatial light modulator. Typically, this requires that each pixel of an imaging sensor be aligned such that it receives light from a corresponding pixel of the spatial light modulator. In this manner, every pixel of the spatial light modulator can be monitored by a pixel of the phase measurement system. As a result, there is a need for a method and system for aligning a spatial light modulator with respect to an imaging device of a phase measurement system.